1. Field of the Invention
The present invention relates to a method for operation of a hearing aid having a microphone system, a signal processing unit and an output transducer, wherein the microphone system has at least two microphone units from which microphone signals originate and which have different-order directional characteristics, and wherein the directional characteristic of the microphone system is variable during operation of the hearing aid. The invention also relates to a hearing aid for Implementing the method.
2. Description of the Prior Art
Modern hearing aids make use of devices for classification of hearing situations. The transmission parameters of the hearing aid are varied automatically depending on the hearing situation. In the process, the classification may influence, inter alia, the method of operation of the interference noise suppression algorithms, and the microphone system. First, as an example, a choice is made on the basis of the identified hearing situation (discretely switched or continuously superimposed) between an omnidirectional directional characteristic (zero-order directional characteristic) and considerable directionality of the microphone system (first order or higher order directional characteristic). The directional characteristic is produced by using gradient microphones or by electrically interconnecting two or more omnidirectional microphones. Microphone systems such as these have a frequency-dependent transmission response, which is characterized by a considerable drop at low frequencies. The noise response at the microphones is also independent of frequency, and is slightly amplified in comparison to an omnidirectional microphone In order to achieve a natural sound impression, the high-pass frequency response of the microphone system has to compensate for this by amplification of the low frequencies. The noise that is present in the low frequency range likewise is amplified in the process and in some circumstances is clearly audible in a disturbing manner, with quieter sounds being concealed by the noise.
German OS 198 49 739 discloses a hearing aid having at least two microphones forming a directional microphone system. In order to avoid undesirable corruption of the directional microphone characteristic resulting from microphones that are not matched to one another, characteristic values of the signals from both microphones are detected by a comparison element, a control element and an actuating element, and are matched to one another in the event of any discrepancy.
PCT Application WO 00/76268 discloses a hearing aid having a signal processing unit and at least two microphones, which can be interconnected to form different order directional microphone systems, in which case the directional microphone systems can themselves be interconnected with a weighting that is dependent on the frequency of the microphone signals emitted from the microphones. The cut-off frequency between adjacent frequency bands in which different weighting of the microphone signals is provided can be adjusted as a function of the result of signal analysis.
European Application 0 942 627 discloses a hearing aid having a directional microphone system with a signal processing device, an earpiece and two or more microphones, the output signals of which can be interconnected via delay elements and the signal processing device with different weighting in order to produce an individual directional microphone characteristic. The preferred reception direction (main direction) can be adjusted individually in the directional microphone system for matching to the existing hearing situation.
U.S. Pat. No. 5,524,056 discloses a hearing aid having an omnidirectional microphone and a first order or higher order directional microphone. The amplitude of the microphone signal from the directional microphone is amplified in the low signal frequency range, and is matched to the microphone signal from the omnidirectional microphone. To produce a frequency response that is as linear as possible, an equalizer is provided in the microphone signal path from the directional microphone, and raises the microphone signal in the lower frequency range. Both the microphone signal from the omnidirectional microphone and the microphone signal from the directional microphone are supplied to a switching unit. The omnidirectional microphone is connected to a hearing aid amplifier when the switching unit is in a first switch position, and the directional microphone is connected to a hearing aid amplifier when the switching unit is in a second switch position. The switching unit can switch automatically as a function of the signal level of a microphone signal.
One disadvantageous feature of the known hearing aids with a directional microphone system is that, when switching between different directional characteristics of the microphone system or when a rapid transition takes place from one directional characteristic to another, this results in sudden level changes and thus artefacts.